Intrinsically safe barrier and field bus system

Abstract

In a field bus system for performing communication between at least one field device and at least one higher-rank apparatus which are connected on a field bus transmission line constituted by a pair of transmission lines, an intrinsically safe barrier disposed between a hazardous area and a safe area on the transmission line in order to limit energy to be used in the hazardous area, the intrinsically safe barrier comprising impedance converting circuits provided in the barrier for reducing impedance in a frequency band of communication data.

What is claimed is: 
     
       1. In a field bus system for performing communication between at least one field device and at least one higher-rank apparatus which are connected on a field bus transmission line constituted by a pair of transmission lines, an intrinsically safe barrier disposed between a hazardous area and a safe area on said transmission line in order to limit energy to be used in said hazardous area, said intrinsically safe barrier comprising impedance converting circuits provided in said barrier for reducing impedance in a frequency band of communication data.   
     
     
       2. An intrinsically safe barrier according to claim 1, wherein said impedance converting circuits are connected in parallel to said transmission lines, respectively, of said transmission line. 
     
     
       3. An intrinsically safe barrier according to claim 2, wherein: said barrier has a structure in which Zener diodes are provided in parallel to each other and between said transmission lines of said transmission line and load resistors are provided in series to said transmission lines of said transmission line respectively; and   said impedance converting circuits are connected in parallel to said load resistors, respectively.   
     
     
       4. An intrinsically safe barrier according to claim 3, wherein each of said impedance converting circuits is constituted by a capacitor, an inductor, and a safety holding part. 
     
     
       5. An intrinsically safe barrier according to claim 1, wherein said impedance converting circuits are connected in parallel to each other and between said transmission lines of said transmission line so as to connect said transmission lines of said transmission line to each other. 
     
     
       6. An intrinsically safe barrier according to claim 5, wherein: said barrier has a structure in which Zener diodes are provided in parallel to each other and between said transmission lines of said transmission line and load resistors are provided in series to said transmission lines of said transmission line respectively; and   said impedance converting circuits are connected at opposite ends of said load resistors respectively, and in parallel to each other between said transmission lines of said transmission line.   
     
     
       7. An intrinsically safe barrier according to claim 1, wherein said impedance converting circuits are composed of a pair of load resistors, a pair of capacitors, a transformer, and a safety holding part. 
     
     
       8. An intrinsically safe barrier according to claim 1, wherein terminators are provided at opposite ends of said transmission line so as to be disposed on a safe area side and on a hazardous area side, respectively, with respect to said barrier. 
     
     
       9. An intrinsically safe barrier according to claim 8, wherein said terminator provided on said safe area side is included in said barrier. 
     
     
       10. A field bus system for performing communication between at least one field device and at least one higher-rank apparatus which are connected on a field bus transmission line constituted by a pair of transmission lines, said field bus system comprising an intrinsically safe barrier disposed between a hazardous area and a safe area on said transmission line in order to limit hazardous area side energy to a predetermined value to thereby reduce impedance in a frequency band of communication data exchanged between said field device and said higher-rank apparatus.   
     
     
       11. A field bus system according to claim 10, further comprising: voltage limiting means provided in said barrier so as to connect said transmission lines of said transmission line;   surplus electric power consumption protecting means and voltage drop elements disposed so as to be connected in series to said transmission lines, respectively, of said transmission line; and   impedance converting circuits connected in parallel with said voltage drop elements respectively to reduce impedance in a frequency band of communication data.   
     
     
       12. A field bus system according to claim 10, further comprising: voltage limiting means provided in said barrier so as to connect said transmission lines of said transmission line;   surplus electric power consumption protecting means and voltage drop elements disposed so as to be connected in series to said transmission lines, respectively, of said transmission line; and   impedance converting circuits connected to opposite ends of said voltage drop elements respectively and connected in parallel to said transmission line so as to connect said transmission lines of said transmission line to thereby reduce impedance in a frequency band of communication data.   
     
     
       13. A field bus system according to claim 10, wherein a signal obtained by superimposing a Manchester code signal pulsated in positive and negative directions with respect to a consumed current for operating said field device is used as a communication signal of said field device. 
     
     
       14. A field bus system according to claim 13, wherein the frequency band of said communication signal depends on the relation between communication rate and the power distribution of the Manchester code. 
     
     
       15. A field bus system according to claim 10, wherein terminators are provided at opposite ends of said transmission line so as to be disposed on a safe area side and on a hazardous area side, respectively, with respect to said barrier. 
     
     
       16. A field bus system according to claim 15, wherein said terminator provided on said safe area side is included in said barrier. 
     
     
       17. A field bus system for performing communication between at least one field device and at least one higher-rank apparatus which are connected on a field bus transmission line constituted by a pair of transmission lines, comprising an intrinsically safe barrier disposed between a hazardous area and a safe area on said transmission line in order to limit energy in said hazardous area to a predetermined value so that said communication is performed through said intrinsically safe barrier, wherein an impedance converting circuit is connected in parallel to said transmission line to reduce impedance in a frequency band of communication data exchanged between said field device and said higher-rank apparatus. 
     
     
       18. A field bus system according to claim 17, wherein said impedance converting circuit is disposed in the outside of said barrier so as to be detachable from said transmission line. 
     
     
       19. A field bus system according to claim 17, wherein said impedance converting circuit is connected in parallel to said barrier. 
     
     
       20. A field bus system according to claim 17, wherein said impedance converting circuit has four connection terminals which are disposed in said barrier and connected to opposite ends of voltage effect elements connected in series to said transmission lines of said transmission line.